1. Field of the Invention
The present invention relates generally to captive screws.
2. Brief Description of the Prior Art
Captive screws are screws that are “captivated” or held within a collar or ferrule. The ferrule is mounted in a first panel or like object. The screw portion is captivated so that it can be moved perpendicular to the first panel and the screw's threads can engage a corresponding aperture in a second panel, so that the first and second panels can be secured together. However, when the panels are unscrewed and disengage, the screw is retained in the first panel. Captive screws are useful in applications where it is important to avoid dropping or losing screws during assembly or repair, such as electronic devices, where a lost screw can cause catastrophic electrical shorts damaging equipment.
One particular application for captive screws is to hold down a heat sink which is mounted on top of a microchip and is intended to dissipate the heat of the microchip. The microchip, in turn, is fastened to a circuit board. Oftentimes, between the heat sink and the microchip a compressible or elastic material in the form of the sheet is placed. If one were to put the heat sink directly on top of the microchip, there would be tiny gaps between the two. Since air conducts heat poorly, these gaps have a detrimental effect on heat transfer. This interface material in the form of a sheet is needed and the sheet has a high thermal conductivity. Recently, microchips during operation have been getting warmer and warmer as space has been getting more and more limited on circuit boards. Accordingly, heat sinks have been getting larger and larger as the requirements for withstanding vibrations has been getting more stringent. Standoffs have been used which rise from the board to which the heat sinks are screwed down. Difficulties encountered with standoffs have been that the standoffs do not compensate for chip height variation and they do not provide consistent compression of the interface material between the chip and the heat sink.
In the past, various clamps, latches, pins, etc have been used to hold heat sinks down. Plastic clips have been used, however, this requires that the use of a screwdriver to screw in the clip directly onto the circuit board which makes the board susceptible to damage. Plastic clips, although they are not costly, are limited by the weight of the heat sink and the clips have proven to be unreliable in vibration and drop shock testing. Some varieties of clips are clipped onto the heat sink at the ends of fins in the middle of the heat sink. Unfortunately, the middle fins are the most important fins in a heat sink as they carry most of the heat load. Therefore, when such clips are used the need for a larger heat sink results.
Accordingly, a need exists for a captive screw which is captive or fixed to a first panel such as a heat sink whereby a collar on the screw bottoms out at a fixed distance on a lower panel when the screw is screwed into the lower panel thereby allowing the captive screw to provide a constant pressure on the top panel. In addition, the captive screw should be able to pass stringent vibration testing and be able to allow for thermal expansion.
The present invention has been developed in view of the foregoing and to overcome the deficiencies of the prior art.